1. Field of the invention
The present invention relates to a method of interpolation and the application thereof to shaft angle encoders.
2. Description of Prior Art
Shaft angle encoders are used in a particularly wide range of applications such as machine tool control systems, antenna servo systems and tachometers. In its most basic form a photo-electric shaft angle encoder comprises a disc with a photo transmission pattern provided thereon a light source and one or more photo-detectors. The disc is fixed concentrically with a shaft and the light source and detector positioned so that light falling on the detector is intercepted by the disc. Rotation of the shaft allows illumination of the detector in accordance with the transmission pattern on the disc. The output of the photo-detector provides information concerning rotation of the shaft in the form of an electric signal. Subsequent processing of the signal depends upon the application in which the encoder is being used but in many applications it is desired to obtain more information than is inherent in the disc pattern alone. In order to achieve this various interpolation techniques are employed.
There are also two basic types of encoder disc which are employed; one containing a number of tracks with each defined sector carrying a unique transmission pattern and the other type of disc often only having a single track in which different sectors can not be distinguished. The first mentioned type of disc is known as an `absolute` encoder disc since each defined unit of angular rotation is uniquely identifiable. The second mentioned type of disc is known as an `incremental` encoder disc since the orientation of the disc is not identifiable from an instantaneous reading of the disc pattern, although an increment in angular rotation is identifiable. It should be noted that variations of the optical arrangement as well as non-optical sensors are also known.
In order to increase the precision with which rotation of the shaft can be determined, and in some instances to enable further information to be deduced, it is common practice to derive two output signals from respective detectors which are offset relative to each other. The output signals, which are necessarily phase related, are used as input for an interpolation technique. The interpolation typically increases the precision of detection of angle of rotation several fold and increases of several thousand fold are obtainable with some systems.
One of the most widely used techniques is based upon a ring of resistive elements into which the two detector signals and their complements are injected. Signals are read from locations diametrically opposite each other on the ring and are fed into a voltage comparator. Several sets of such signals are read from the ring and each fed into a respective comparator. The comparator outputs are capable, after further processing, of a several fold increase in resolution of angular detection of rotation. To obtain a five fold increase approximately 20 resistive elements might be required within the ring, with perhaps another 40 being required in comparator circuits. If the initial detector signals are not sinusoidal then the resistive elements will need to be of non-equal values. The comparators may have significant offset voltages of a non-standard nature and these cause variations in output with changes of temperature.
Another prior arrangement, known as an `Optical Resolver`, makes use of phase shifted signals read from the encoder disc. A system clock drives a counter, typically of 7 bits from which sequential interrogation signals are derived. These interrogation signals are used to sample each of the two disc generated analogue signals. After combining these sampled signals, the resultant is passed into a filter tuned to the count cycle frequency. The filter output is a sinewave, the phase of which varies with respect to the system clock cycle according to the relative amplitudes of the analogue inputs. A zero-crossing detector circuit acting on the filter output produces a timing pulse which transfers the instantaneous counter word into an output register. So long as the disc remains stationary, the same word will be transferred from the system clock counter to the output.
A small change in disc angle, and hence in the relative amplitudes of the two analogue signals, changes the filter output phase with respect to the system clock, and the output register receives a different word from the counter representative of the new shaft angle.
Thus, the output word varies as the shaft rotates. This arrangement gives good accuracy but has a particularly restricted upper limit upon speed of shaft rotation, due to the frequency of operation of its circuitry especially at the filtering and counting stages. In addition, complex circuitry is required and many of the components require extremely careful selection.
A further prior technique relies upon two phase shifted signals being read from the encoder disc and applied to respective deflection plates of a Cathode Ray Tube(CRT). The effect of the signals is to cause the CRT spot to describe a circuitous path on the screen, completing one revolution for each `cycle` of the disc pattern. A stationary mechanical filter is placed between the CRT screen and a photo-detector. Such that the CRT display periodically illuminates the photo-detector. This produces a signal comprising pulses at a frequency equal to the resolution of the mechanical filter multiplied by the number of disk cycles. Thus an incremental encoder of significantly improved precision may be achieved. However, this technique is not well suited to commercial applications due to the relative expense and fragile nature of the CRT.